I have two. They are not marked in anyway. My two had slighly different receiver extensions. They are heavy around 0.960" under the handguards. The extensions are pretty tight going into the upper but do go. The muzzle is properly threaded 1/2"-28tpi for 5/8". They are NOT stainless, they are in-the-white steel, probably 4140. One of mine had just a bit of surface rust when it showed up.
I purchased a couple used FSB's off the EE and drilled and reamed the barrel for 2/0 taper pins. This worked out ok but I would use new undrilled FSB's if I were to do it again as it's hard to hit the opposite side of the FSB even using my mini-mill. I just blasted and parked them today after work.
I'll be putting them on the EE for sale at some point. Speaking of which, I have seen a few of these showing up on the EE for sale.
I bought them as a project to try FSB work. If I were to build one up I would use a undrilled FSB or clamp-on/setscrew gasblock/FSB.